Pacing instruments can be used to treat patients suffering from any of a number of heart conditions, such as a reduced ability to deliver sufficient amounts of blood from the heart. For example, some heart conditions may cause or be caused by conduction defects in the heart. These conduction defects may lead to irregular or ineffective heart contractions. Cardiac pacing systems (e.g., a pacemaker or an implantable defibrillator with pacing capability) may be implanted in a patient's body so that wire electrodes in contact with the heart tissue provide electrical stimulation to regulate electrical conduction in the heart tissue. Such regulated electrical stimulation is done to cause the heart to contract and hence pump blood.
The wired pacing systems in current use include a pulse generator that is implanted, typically in a patient's pectoral region just under the skin. One or more wired leads extend from the pulse generator so as to contact various portions of the heart. An electrode at a distal end of a lead may provide the electrical contact to the heart for delivery of the electrical pulses generated by the pulse generator and delivered to the electrode through the lead.
The use of wired leads may limit the number of sites of heart tissue at which electrical energy may be delivered. For example, most commercially available pacing leads are not indicated for use inside the left chambers of the heart. One reason is that the high pumping pressure in the left chambers of the heart may cause a thrombus or clot that forms on the bulky wired lead to eject into distal arteries, thereby causing stroke or other embolic injury. Thus, in order to pace the left side of the heart with a wired lead, most wired leads are directed through the cardiac venous system (outside the left chambers of the heart) to a site in a cardiac vein along the exterior of the left side of the heart.
In one example of a pacing therapy that includes pacing of a left heart chamber, a treatment known as biventricular pacing may be performed when the left ventricle does not contract in synchrony with the right ventricle. In order to perform such pacing therapy, typically a first wired lead is implanted through a vein into the right atrium, a second wired lead is implanted through a vein into the right ventricle, and a third wired lead is implanted through a vein and into the coronary sinus vein (to pace the left ventricle wall from outside the left ventricle). These three wired leads may be connected to a pacemaker device (e.g., implanted in the pectoral region) in an attempt to regulate the contractions of the right and left ventricles.
In addition to conventional wired pacing systems, a new class of pacing system is being developed that includes wireless operation. In such systems, a control module wirelessly communicates with electrode assemblies that are implanted along the outside of the heart tissue or embedded in a cardiac vein. The wireless communication from the control module can provide a source of power through inductive coupling to the implanted electrode assembly. One design issue for such wireless pacing systems is the efficiency of the inductive coupling between the control module and the implanted electrode assemblies, which can impact the battery life of the control module. For example, the further an implanted electrode assembly is away from the control module, the greater the power requirement for the control module to communicate with the implanted electrode. The power draw from the control module battery can be significant when inductively coupling with an implanted electrode assembly disposed on a distant portion of the heart.